Ashley Olsen (right) is reportedly battling late-stage Lyme disease, which she contracted through a tick bite.

Ashley Olsen is battling late-stage Lyme disease, which she reportedly contracted through a tick bite. Ashley and sister Mary-Kate Olsen will not return for the “Fuller House” reunion slated for next year.

Ashley, 28, was diagnosed in the late stages of the incurable bacterial infection and has progressively gotten worse, Examiner reported.

“She’s really sick,” a source told OK! “It’s gotten worse. She was diagnosed in the very late stages, so early detection measures weren’t options for her. She’s really going through it. When she does come to work, she looks haggard and dishevelled. And she’s often moody.”

Other celebrities who have been struggling with Lyme disease are pop star Avril Lavigne and “Real Housewives of Beverly Hills” star Yolanda Foster, who was diagnosed with Lyme in 2012. Yolanda recently got a stem cell transplant to treat her illness, according to Examiner.

In January, Yolanda revealed she no longer has the ability to read, write or watch TV as a result of her illness.

“I have lost the ability to read, write, or even watch TV, because I can’t process information or any stimulation for that matter,” said Yolanda, who encouraged everyone to participate in the Lyme Disease Challenge to raise awareness and funding.